Question
I live in the Philadelphia, Pa. area.
I have a retaing wall that is 2 ft. high.
On top of the wall i have about 1 ft. of dirt,with stone underneath for wall drainage.
I am looking for a Perennial that is round in shape & grows to about 18 in height & width.
I saw some while driving around,but don't know what they are called.
Can you recommend some for me.

Answer
I'm going to need some more information, I have a few in mind but to know if they will do well I have to know:
What is the light situation?  Full sun?  Part shade?  Trees?
Is there a drip line or some irrigation that it can recieve?
Are there plants around it?
Low maintenence?
Wind?  Exposure to other elements?

Once I know all about the site and microclimate I can answer your question.  It sounds as though you do have good drainage, which is great for any plant.  Pictures would help as well, take your camera for a drive and snap off a couple shots at some things you like and I can identify them as well, if you can't attach them to this forum you can e-mail them to me at [email protected].  Good luck!  Hopefully I'll be talking to you again soon with some answers for you.

_Jason